Elastomeric seals are in very common use in a wide variety of applications as a means for closing off a flow passageway (gap) between two parts. The parts are usually metallic and will, unless measures are taken, allow fluids to pass through the gap where the two pieces are joined. To prevent the escape or loss of fluid at these gaps, flexible elastomeric seals are typically used to close the gap between the two parts. To achieve this function, the elastomeric seal is placed in a cavity or groove in a first part and the exposed side of the seal is comated with the surface of a second part. The prevention of fluid passage through a gap between such parts generally relies upon the maintenance of an initial interference fit of the seal with attendant interface biasing forces between the sealing element and the two parts.
Previously this initial interference fit, which is termed ‘presqueeze’ and refers to the condition prior to the application of fluid pressure, has been obtained either: a) passively from displacement-induced forces due to the size and protrusion of the elastomeric seal when mounted in the groove, or b) actively by compressing the elastomeric seal after it is mounted in the groove. Sanders et al. U.S. Pat. No. 5,437,489 shows examples of passively presqueezed seals, while Reneau U.S. Pat. No. 4,728,125 discusses an example of an actively presqueezed seal.
As fluid pressure is applied to one side of the elastomeric seal, the seal will deform and shift in the direction of the fluid pressure forces. With time under high pressure loads and/or as the pressure increases, the seal will continue to displace toward the low pressure side of the groove and become further distorted and “cold flow” or “creep” into the gap. This time-dependent behavior is further enhanced if the elastomeric seal shrinks in volume or is softened by heat or its interaction with retained fluids. This problem is intensified when the elastomeric material begins to shear off into the gap to be sealed. In some cases the entire seal is displaced into the gap. Shearing and tearing of the elastomeric material from the extrusion of the seal into the gap can cause the seal to fail. These problems are significantly amplified as the size of the gap to be sealed is increased.
The industry has implemented a number of improvements in seals to help solve the problems of creep and extrusion, which lead to seal failure. Such improvements have enhanced elastomeric seal performance, but none of the improvements have fully solved the problem of creep and extrusion, particularly for large gaps and for high pressure situations.
A frequent improvement used for large gap or high-pressure situations has been to provide an antiextrusion device on the low-pressure side of the seal. This approach can minimize static and creep deflections of the seal into the seal gap. The typical antiextrusion device is made of a stiffer, stronger material than the seal elastomer. The antiextrusion device is either integrally bonded to the external surface of the seal or retained in the seal groove as a separate item. Either way, the antiextrusion device is generally positioned on the downstream face of the seal to protrude into the gap and back up the seal. Antiextrusion devices assist in reducing sensitivity of the elastomer seal to creep, thereby aiding in the maintenance of the initial interference fit.
The antiextrusion device ideally should provide low resistance to distortion (i.e., low stiffness) across the seal gap to permit large deflections of the device in that direction without the device undergoing permanent deformation. Concurrently, the antiextrusion device must provide both high stiffness and high strength to resist bending and shear distortion of the seal element into the gap. Sealing the gap and resisting creep of the seal into the gap requires some embedment or entrapment of the antiextrusion device in the seal to permit the seal to react against the low-pressure end wall of the seal groove to provide resistive forces to pressure loading. These requirements are very difficult to satisfy for linear, annular or circumferential seals for large gaps, because provision of adequate stiffness and strength for resisting movement into the gap generally requires that the antiextrusion device (ring) be provided with a geometry which causes the ring to have undesirably high resistance to distortion across the gap. Generally, only a very limited gap size can be spanned by currently used antiextrusion devices without permanent distortion of the devices.
Two types of non-integral, metallic antiextrusion devices are used for large gaps for both linear and annular seals. One type uses non-integral, bendable metallic fingers on the downstream side of the seal. These fingers have a common base strip which serves as anchor, while each finger functions independently. In certain antiextrusion rings of this type, the individual metallic fingers undergo excess bending and are not reliable for multiple sealings. In fact, they have been known to evert due to inadequate bending strength or excessive gap in severe cases. The second type of non-integral, metallic antiextrusion rings are knitted metal annular antiextrusion rings (Metex, Edison, N.J.). These knitted metal rings are suitable for relatively large gaps and are used for oilfield downhole packers. However, these knitted antiextrusion rings have very little elastic rebound, so that resetting of the seal is not advisable or necessarily feasible due to inability to fully retract.
The use of antiextrusion rings made of more flexible materials, such as a stiff elastomer or plastic material, for large circumferential seal gaps requires that the size of the antiextrusion ring and seal be significantly increased in order to provide sufficient embedment of the antiextrusion ring to resist creep, bending, and shearing of the rings. For active mechanically compressed seals, such as in Reneau U.S. Pat. No. 4,728,125 or the Oceaneering “Smart Flange Plus”™ (Oceaneering International, Inc., Houston, Tex.), the larger rings and seals require larger seal compression hardware and a significantly larger and much more expensive housing. Again, provision of satisfactory resistance to bending distortion in the seal gap will impede the ability of the antiextrusion ring to adequately distort to span a large gap. Stiffer ring materials have improved creep and stiffness performance, but are less conformable to large gaps and generally will permanently distort when spanning larger gaps. Less stiff ring materials require even larger seal cavities to adequately embed them.
The significant areas of performance difficulty cited for large gaps and high pressures with conventional seals frequently lead to leaks or complete seal failures. For critical service conditions, such as deep water subsea pipeline repair clamps or hot-tap pipeline fittings, revisiting the clamp for adjusting the compressional preload on installed seals is prohibitively expensive. Further, providing more compressional preload in such cases is not practical for passive seals for reasons of installation damage to the seal due to excessive interference and an increased tendency of the seal to creep and extrude through the gap with high preloads.
Thus, a need exists for seals that can perform in large gap and high pressure situations.